Linear Ether Electrolyte and Asymmetric End Groups for Use in Lithium Batteries

ABSTRACT

A primary electrochemical cell and electrolyte incorporating a linear asymmetric ether is disclosed. The ether may include EME, used in combination with DIOX and DME, or have the general structural formula R 1 —O—CH 2 —CH 2 —O—R 2  or R 1 —O—CH 2 —CH(CH 3 )—O—R 2 , where a total of at least 7 carbon atoms must be present in the compound, and R 1  and R 2  consist alkyl, cyclic, aromatic or halogenated groups but cannot be the same group (i.e., R 1 ≠R 2 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/111,520 filed on Apr. 29, 2008, which is fully incorporated herein byreference.

BACKGROUND OF INVENTION

This invention relates to a nonaqueous electrolyte for a primaryelectrochemical cell, such as a lithium/iron disulfide cell. Morespecifically, a ternary electrolyte including dioxolane, dimethoxyethaneand a linear ether with asymmetric end groups is contemplated.

Batteries are used to provide power to many portable electronic devices.In today's consumer-driven device market, standardized primary cellsizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V)are preferred. Moreover, consumers frequently opt to use primarybatteries for their low cost, convenience, reliability and sustainedshelf life as compared to comparable, currently available rechargeable(i.e., secondary) batteries. Primary lithium batteries (those thatcontain metallic lithium or lithium alloy as the electrochemicallyactive material of the negative electrode) are becoming increasinglypopular as the battery of choice for new devices because of trends inthose devices toward smaller size and higher power.

One type of lithium battery that is particularly useful for 1.5 Vconsumer devices is the lithium-iron disulfide (or LiFeS₂) battery,having the IEC designations FR6 for AA size and FR03 for AAA size.LiFeS₂ cells offer higher energy density, especially at high drain ratesin comparison to alkaline, carbon zinc or other primary (i.e.,non-rechargeable) battery systems. Such batteries use iron disulfide,FeS₂ (also referred to as pyrite or iron pyrite, which the preferredmineral form of iron disulfide for battery applications), as theelectrochemically active material of the positive electrode.

As a general rule, the electrolyte in any battery must be selected toprovide sufficient electrical conductivity to meet the cell dischargerequirements over the desired temperature range. As demonstrated by U.S.Pat. No. 4,129,691 to Broussely, increasing the solute (i.e., salt)concentration in a lithium battery electrolyte is expected to result ina corresponding increase in the conductivity and usefulness of thatelectrolyte. However, other limitations—such as the solubility of thesolute in specific solvents, the formation of an appropriate passivatinglayer on lithium-based electrodes and/or the compatibility of thesolvent with the electrochemically active or other materials in thecell—make the selection of an appropriate electrolyte system difficult.As a non-limiting example, U.S. Pat. No. 4,804,595 to Bakos describeshow certain ethers are not miscible with solvents such as propylenecarbonate. Additional electrolyte deficiencies and incompatibilities arewell known and documented in this art, particularly as they relate toLiFeS₂ cells and lithium's reactivity with many liquids, solvents andcommon polymeric sealing materials.

Ethers are often desirable as lithium battery electrolyte solventsbecause of their generally low viscosity, good wetting capability, goodlow temperature discharge performance and good high rate dischargeperformance, although their polarity is relatively low compared to someother common solvents. Ethers are particularly useful in cells withpyrite because they tend to be more stable as compared to higher voltagecathode materials in ethers, where degradation of the electrode surfaceor unwanted reactions with the solvent(s) might occur (e.g.,polymerization). Among the ethers that have been used in LiFeS₂ cellsare 1,2-dimethoxyethane (“DME”) and 1,3-dioxolane (“DIOX”), whether usedtogether as taught by U.S. Pat. No. 5,514,491 or 6,218,054 or EuropeanPatent 0 529 802 B1, all to Webber, or used in whole or in part as ablend of solvents as suggested by U.S. Pat. Nos. 7,316,868 to Gorkovenko(use of DIOX and 5-6 carbon 1,3-dialkoxyalkanes); 3,996,069 toKronenberg (use of 3-methyl-2-oxazolidone and DIOX and/or DME); or U.S.Patent Publication No. 2008/0026296A1 to Bowden (use of sulfolane andDME).

Other solvents not specifically containing DIOX or DME may also bepossible, such as those disclosed in U.S. Pat. No. 5,229,227 to Webber(use of 3-methyl-2-oxazolidone with polyalkylyene glycol ethers such asdiglyme). However, because of interactions among solvents, as well asthe potential effects of solutes and/or electrode materials on thosesolvents, ideal electrolyte solvent blends and the resulting dischargeperformance of the cell are often difficult to predict without actuallytesting the proposed blend in a functioning electrochemical cell.

Another class of ethers has been proposed for use as electrolytes, asdisclosed in U.S. Pat. No. 7,316,868. DIOX is used in the blend but theDME is preferentially replaced by one or more 1,2- or1,3-dialkoxyalkanes having 5 or 6 carbon atoms, such as1-ethoxy-2-methoxyethane (“EME”), 1-methoxy-2-propoxyethane,1,2-dimethoxypropane, 1-ethoxy-2-methoxypropane,2-ethoxy-1-methoxypropane, 1,3-dimethoxypropane, and1,3-dimethoxybutane. The resulting solvent blend is expected to haveparticular utility in enhancing the cycle life of lithium-sulfurbatteries specifically in comparison to previously known electrolytescontaining DME instead of EME (see, e.g., Table 2).

A wide variety of solutes has been used in LiFeS₂ cell electrolytes,including lithium iodide (LiI), lithium trifluoromethanesulfonate(LiCF₃SO₃ or “lithium triflate”), lithium bistrifluoromethylsulfonylimide (Li(CF₃SO₂)₂N or “lithium imide”), lithium perchlorate (LiClO₄),lithium hexafluoroarsenate (LiAsF₆) and others. While electrolytescontaining lithium triflate can provide fair cell electrical anddischarge characteristics, such electrolytes have relatively lowelectrical conductivity. Furthermore, lithium triflate is relativelyexpensive. Lithium iodide (LiI) has been used as an alternative tolithium triflate to both reduce cost and improve cell electricalperformance, as discussed in the previously identified U.S. Pat. No.5,514,491 to Webber. One particular brand of AA−-sized FROG batteriessold by Energizer Holdings Inc. currently uses a nonaqueous electrolytewith 0.75 molar concentration of LiI salt in a solvent mixturecontaining DIOX and DME.

Lithium iodide and lithium triflate salts have been used in combinationto provide improved low temperature discharge performance, as describedin related U.S. Patent Publication No. 2006/0046154 to Webber. Asdiscussed therein, LiFeS₂ cells with a high ether content and LiI as asolute (either the sole solute or in combination with lithium triflate)may sometimes, on high rate discharge at low temperatures, exhibit arapid drop in voltage at the beginning of discharge. The voltage candrop so low that a device being powered by the cell will not operate.Eliminating LiI as a solute and making lithium triflate the sole solutecan solve this problem, but the operating voltage can then be too low onhigh rate and high power discharge at room temperature. And the use ofperchlorates as the sole, primary salt or even as a co-salt may beproblematic because of the potential health and safety issues posed bythese compounds.

Additives may be employed in the electrolyte to enhance certain aspectsof a cell and/or its performance. For example, U.S. Pat. No. 5,691,083to Bolster describes the use of a very low concentration of potassiumsalt additives to achieve a desired open circuit voltage in cells with acathode material including FeS₂, MnO₂ or TiS₂. U.S. Publication No.2008/0026290 to Jiang discloses the use of an aluminum additive to slowthe development of a passivation film on the surface of the lithiumelectrode. In each of these examples, the benefit of the additive(s)selected must be balanced against any deleterious reactions or effects(in terms of discharge performance, safety and longevity of thebattery).

Finally, as mentioned above, it is believed higher concentrations ofsolute(s) normally improve the conductivity of the electrolyte. However,certain systems (typically in rechargeable lithium-sulfur batterysystems where non-chalcogenic polysulfides are the preferred cathodematerial) utilize a “catholyte” where portions of the electrode itselfdissolve into the electrolyte solution to provide ionic conductivity. Insuch systems, minimal to non-existent concentrations of lithium ions maybe provided to a fully charged cell without compromising performance astaught by U.S. Pat. No. 7,189,477 to Mikhaylik. Ultimately, LiFeS₂ andother lithium electrochemical cells do not exhibit this propensity toprovide ions from the electrodes to the electrolyte, thereby eliminatingthe usefulness of this approach in LiFeS₂ systems and more generallyillustrating the pitfalls associated with blindly applying teachingsfrom a given electrochemical system to another, dissimilar system.

SUMMARY OF INVENTION

An electrolyte consisting of one or more solutes, such as lithium iodideand/or other common salts, dissolved in a nonaqueous, organic solventblend consisting essentially of 1,3-dioxolane, 1,2-dimethoxyethane and1-ethoxy-2-methoxyethane is contemplated. The DME and EME must beprovided at a level of at least 10 vol. % each. Preferably, the DIOX isprovided at greater than 40 vol. %. In some embodiments, it is preferredto provide the DME in twice the volume of the EME present in the blend.In other embodiments, the DME and/or EME are provided as 10-30 vol. % ofthe overall solvent blend.

A lithium-iron disulfide electrochemical cell is also contemplated. Thecell has a lithium-based anode, an iron disulfide-based cathode and anelectrolyte comprising DIOX, DME and EME, where DME and EME eachconstitute at least 10 vol. % of the solvent blend used in theelectrolyte. As above, 10-30 vol. % of DME and/or EME is possible, andthe DIOX preferably constitutes over half of the solvent blend byvolume. The solute may include lithium iodide, although other salts arecontemplated. The resulting cell exhibits superior low temperatureperformance as compared to those known in the art.

Finally, a primary electrochemical cell with a linear asymmetric etherelectrolyte is contemplated. The electrolyte includes at least onesolute dissolved in a solvent consisting essentially of at least 40 vol.% of DIOX, at least 10 vol. % of one linear asymmetric ether and anoptional amount of DME. The asymmetric ether(s) is/are selected from thegroup consisting of: EME, a first compound with the formulaR₁—O—CH₂—CH₂—O—R₂ and a second compound with the formulaR₁—O—CH₂—CH(CH₃)—O—R₂. However, for both the first and second compounds,a total of at least 7 carbon atoms must be present in the compound, andR₁ and R₂ consist alkyl, cyclic, aromatic or halogenated groups butcannot be the same group (i.e., R₁≠R₂).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross sectional view of a LiFeS₂ battery according to oneembodiment of the invention.

DETAILED DESCRIPTION OF INVENTION

Unless otherwise specified, as used herein the tetras listed below aredefined and used throughout this disclosure as follows:

-   -   ambient (or room) temperature—between about 20° C. and about 25°        C.; unless otherwise stated, all examples, data and        manufacturing information were provided/conducted at ambient        temperature.    -   anode—the negative electrode; more specifically, within the        meaning of the invention, it consists essentially of lithium or        an alloy containing at least 90% lithium by weight as the        primary electrochemically active material.    -   cathode—the positive electrode; more specifically, within the        meaning of the invention, it comprises iron disulfide as the        primary electrochemically active material, along with one or        more rheological, polymeric and/or conductive additives, coated        onto a metallic current collector.    -   cell housing—the structure that physically encloses the        electrochemically active materials, safety devices and other        inert components which comprise a fully functioning battery;        typically consists of a container (formed in the shape of a cup,        also referred to as a “can”) and a closure (fitting over the        opening of the container, typically consists of venting and        sealing mechanisms for impeding electrolyte egress and        moisture/atmospheric ingress).    -   DIOX—a dioxolane-based solvent, typically 1,3-dioxolane    -   DME—a dimethoxyethane-based solvent, typically        1,2-dimethoxyethane    -   electrolyte—one or more solutes dissolved within one or more        liquid, organic solvents; does not include any electrochemical        systems where the cathode is expected to partially or completely        dissolve in order to contribute ionic conductivity to the cell        (i.e., a “catholyte” such as those utilized in lithium-sulfur        batteries)    -   EME—an ethoxy-methoxyethane-base solvent, typically 1-ethoxy,        2-methoxyethane    -   jellyroll (or spirally wound) electrode assembly—strips of anode        and cathode, along with an appropriate polymeric separator, are        combined into an assembly by winding along their lengths or        widths, e.g., around a mandrel or central core.    -   nominal—a value, specified by the manufacturer, that is        representative of what can be expected for that characteristic        or property.    -   percent discharge—the percentage of the capacity removed from a        cell as a result of its intended use, but excluding capacity        removed by deliberate conditioning or preliminary discharge        performed by a manufacturer to make the cell more suitable for        consumer use.    -   salt—as part of the electrolyte, an ionizable compound,        typically including lithium or some other metal, dissolved in        one or more solutes.

Cell Design

The invention will be better understood with reference to FIG. 1, whichshows a specific cell design that may be implemented. Cell 10 is an FR6type cylindrical LiFeS₂ battery cell, although the invention should haveequal applicability to FR03 or other cylindrical cells. Cell 10 has ahousing that includes a container in the form of a can 12 with a closedbottom and an open top end that is closed with a cell cover 14 and agasket 16. The can 12 has a bead or reduced diameter step near the topend to support the gasket 16 and cover 14. The gasket 16 is compressedbetween the can 12 and the cover 14 to seal an anode or negativeelectrode 18, a cathode or positive electrode 20 and electrolyte withinthe cell 10.

The anode 18, cathode 20 and a separator 26 are spirally wound togetherinto an electrode assembly. The cathode 20 has a metal current collector22, which extends from the top end of the electrode assembly and isconnected to the inner surface of the cover 14 with a contact spring 24.The anode 18 is electrically connected to the inner surface of the can12 by a metal lead (or tab) 36. The lead 36 is fastened to the anode 18,extends from the bottom of the electrode assembly, is folded across thebottom and up along the side of the electrode assembly. The lead 36makes pressure contact with the inner surface of the side wall of thecan 12. After the electrode assembly is wound, it can be held togetherbefore insertion by tooling in the manufacturing process, or the outerend of material (e.g., separator or polymer film outer wrap 38) can befastened down, by heat sealing, gluing or taping, for example.

An insulating cone 46 is located around the peripheral portion of thetop of the electrode assembly to prevent the cathode current collector22 from making contact with the can 12, and contact between the bottomedge of the cathode 20 and the bottom of the can 12 is prevented by theinward-folded extension of the separator 26 and an electricallyinsulating bottom disc 44 positioned in the bottom of the can 12.

Cell 10 has a separate positive terminal cover 40, which is held inplace by the inwardly crimped top edge of the can 12 and the gasket 16and has one or more vent apertures (not shown). The can 12 serves as thenegative contact terminal. An insulating jacket, such as an adhesivelabel 48, can be applied to the side wall of the can 12.

Disposed between the peripheral flange of the terminal cover 40 and thecell cover 14 is a positive temperature coefficient (PTC) device 42 thatsubstantially limits the flow of current under abusive electricalconditions. Cell 10 also includes a pressure relief vent. The cell cover14 has an aperture comprising an inward projecting central vent well 28with a vent hole 30 in the bottom of the well 28. The aperture is sealedby a vent ball 32 and a thin-walled thermoplastic bushing 34, which iscompressed between the vertical wall of the vent well 28 and theperiphery of the vent ball 32. When the cell internal pressure exceeds apredetermined level, the vent ball 32, or both the ball 32 and bushing34, is forced out of the aperture to release pressurized gases from thecell 10. In other embodiments, the pressure relief vent can be anaperture closed by a rupture membrane, such as disclosed in U.S. PatentApplication Publication No. 2005/0244706, herein fully incorporated byreference, or a relatively thin area such as a coined groove, that cantear or otherwise break, to form a vent aperture in a portion of thecell, such as a sealing plate or container wall.

The terminal portion of the electrode lead 36, disposed between the sideof the electrode assembly and the side wall of the can, may have a shapeprior to insertion of the electrode assembly into the can, preferablynon-planar that enhances electrical contact with the side wall of thecan and provides a spring-like force to bias the lead against the canside wall. During cell manufacture, the shaped terminal portion of thelead can be deformed, e.g., toward the side of the electrode assembly,to facilitate its insertion into the can, following which the terminalportion of the lead can spring partially back toward its initiallynon-planar shape, but remain at least partially compressed to apply aforce to the inside surface of the side wall of the can, thereby makinggood physical and electrical contact with the can.

Electrolyte

A nonaqueous electrolyte, containing water only in very small quantitiesas a contaminant (e.g., no more than about 500 parts per million byweight, depending on the electrolyte salt being used), is deposited intothe cell housing during manufacture. Because the electrolyte is theprimary media for ionic transfer in a LiFeS₂ cell, selection of anappropriate solvent and solute combination is critical to optimizing theperformance of the cell. Moreover, the solute and solvents selected forthe electrolyte must possess appropriate miscibility and viscosity forthe purposes of manufacture and use of the resulting cell, while stilldelivering appropriate discharge performance across the entire spectrumof temperatures potentially experienced by batteries (i.e., about −40°C. to 60° C.). Furthermore, the electrolyte must be non-reactive andnon-volatile (or at least possess a low enough boiling point to bepractically retained by conventional polymeric seals and closuremechanisms).

Miscibility and viscosity of the solvents and the electrolyte is key tothe manufacturing and operational aspects of the battery. All solventsused in the blend must be completely miscible to insure a homogeneoussolution. Similarly, in order to accommodate the requirements of highvolume production, the solvents must possess a sufficiently lowviscosity to flow and/or be dispensed quickly.

Additionally, the solvents and the electrolyte must possess a boilingpoint appropriate to the temperature range in which the battery willmost likely be exposed and stored (i.e., −40° C. to 60° C.). Morespecifically, the solvent(s) must be sufficiently non-volatile to allowfor safe storage and operation of the battery within this statedtemperature range. Similarly, the solvents and the electrolyte must notreact with the electrode materials in a manner that degrades theelectrodes or adversely affects performance of the battery upondischarge. Suitable organic solvents that have been or may be used inLiFeS₂ cells have included one or more of the following: 1,3-dioxolane;1,3-dioxolane based ethers (e.g., alkyl- and alkoxy-substituted DIOX,such as 2-methyl-1,3-dioxolane or 4-methyl-1,3-dioxolane, etc.);1,2-dimethoxyethane; 1,2-dimethoxyethane-based ethers (e.g., diglyme,triglyme, tetraglyme, ethyl glyme, etc.); ethylene carbonate; propylenecarbonate; 1,2-butylene carbonate; 2,3-butylene carbonate; vinylenecarbonate; methyl formate; γ-butyrolactone; sulfolane; acetonitrile;N,N-dimethyl formamide: N,N-dimethylacetamide;N,N-dimethylpropyleneurea; 1,1,3,3-tetramethylurea; beta aminoenones;beta aminoketones; methyltetrahydrofurfuryl ether; diethyl ether;tetrahydrofuran (“THF”); 2-methyl tetrahydrofuran;2-methoxytetrahydrofuran; 2,5-dimethoxytetrahydrofuran;3,5-dimethylisoxazole (“DMI”); 1,2-dimethoxypropane (“DMP”); and1,2-dimethoxypropane-based compounds (e.g., substituted DMP, etc.).

Salts should be nearly or completely soluble with the selectedsolvent(s) and, as with the discussion of solvent characteristics above,without any degradation or adverse effects. Examples of typical saltsused in LiFeS₂ cells include LiI (“lithium iodide”), LiCF₃SO₃ (“lithiumtriflate”), LiClO₄ (“lithium perchlorate”), Li(CF₃SO₂)₂N (“lithiumimide”), Li(CF₃CF₂SO₂)₂N and Li(CF₃SO₂)₃C. Other potential candidatesare lithium bis(oxalato)borate, lithium bromide, lithiumhexafluorophosphate, potassium hexafluorophosphate and lithiumhexafluoroarsenate. Two key aspects of salt selection are that they donot react with the housing, electrodes, sealing materials or solventsand that they do not degrade or precipitate out of the electrolyte underthe typically expected conditions to which the battery will be exposedand expected to operate (e.g., temperature, electrical load, etc.). Itis possible to use more than one solute to maximize certain aspects ofperformance.

Notably, unless noted to the contrary, the concentration of the solutesrelative to the solvents as described herein is best expressed as molesof solute per kilogram of solution (molality). Molality of a solutionremains constant irrespective of the physical conditions liketemperature and pressure, whereas volume of some solvents typicallyincreases with in temperature thereby yielding a decrease in molarity(i.e., moles per liter). Nevertheless, at ambient temperatures, thedifference between molality and molarity may be negligible.

In order to sustain sufficient service across the entire spectrum oftemperatures (−40° C. to 60° C. or greater), a ternary solvent blendcomprising DIOX, DME and EME was developed. Notably, unlike DME, the endgroups for EME (i.e., the alkyl groups on the opposite terminal ends ofthe ether chain) are not identical:

EME structure: CH₃—O—CH₂—CH₂—O—C₂H₅

DME structure: CH₃—O—CH₂—CH₂—O—CH₃

It is believed that these asymmetric end groups on a linear ether, whendispersed in a mixture of DIOX and DME, should reduce the melting pointof the overall solvent blend, thereby enabling improved low temperatureperformance for LiFeS₂ cells. In essence, the EME acts as a cosolventadditive. Furthermore, it is believed that any asymmetry in the linearether additive could potentially display the desired characteristics.For example, 1-ethoxy-2-methoxypropane (“DMP”) could also be used. Infact, any linear ether of the general formula R₁—O—CH₂—CH₂—O—R₂ orR₁—O—CH₂—CH(CH₃)—O—R₂ is a potential solvent, so long as R₁≠R₂ and theentire compound has at least 7 carbon atoms in total. For example, R₁and R₂ could be short alkyl (e.g., methy, ethyl, propyl, etc.), cyclic,aromatic and/or halogenated (e.g., fluorinated, chlorinated, etc.)groups.

DIOX and DME are preferred solvents. At least greater than 10 volumepercent of DME should be provided, with the balance being DIOX and athird co-solvent, such as EME. More preferably, DME should be providedin an amount that is between one half to twice the amount of EMEprovided, again with the balance being DIOX.

EME is preferred as a third cosolvent, in addition to DME. EME should beprovided as at least 10 volume percent of the solvent or, morepreferably, as at least 15 volume percent. Up to 30 volume percent orgreater of EME could be used while still demonstrating the benefits ofthis invention.

Other linear, asymmetric ethers can be used as the third co-solvent, incombination with DIOX and DME, or possibly as a solvent in combinationwith DIOX only. These linear asymmetric ethers are selected based on thefollowing criteria: (i) the base compound has a structure/formula ofeither R₁—O—CH₂—CH₂—O—R₂ or R₁—O—CH₂—CH(CH₃)—O—R₂. In either case, R₁and R₂ can be alkyl (e.g., methy, ethyl, propyl, etc.), cyclic, aromaticand halogenated (e.g., fluorinated, chlorinated, etc.) groups, providedthat R₁ is not the same group as R₂ and further provided that thecompound, when considered as a whole, has at least 7 or more carbonatoms. As a non-limiting example, in the event thatR₁—O—CH₂—CH(CH₃)—O—R₂ is the structure, R₁ could be a methyl (—CH₃)group and R₂ could be isopropyl (—CH(CH₃)₂) group. Other combinationsare possible, with aromatic and cyclic groups incorporating non-carbonatoms as a constituent of the ring structure specifically included aspotential candidates. In this case, DIOX again should form at least 40vol. % of the solvent blend. DME, EME, DMP or other similar solvents canbe used in combination with these 7+carbon linear, asymmetric ethers.

Lithium iodide is the preferred solute, although other solutes providedto this solvent blend would be expected to exhibit similar benefits(including but not limited to lithium perchlorate, lithium triflate,lithium imide and the like). The preferred solute concentration is 0.75molal.

When this electrolyte is used in conjunction with a LiFeS₂ batteryaccording to the configuration described above, extraordinaryimprovements are observed at extremely low temperatures (i.e., below−20° C.). In many embodiments, double the amount of capacity can beexpected. Moreover, these improvements can be realized withoutsacrificing performance at room temperature or on specialized dischargetests, such as the American National Standard Institute (ANSI) digitalstill camera pulse test.

Other Cell Components

The cell container is often a metal can with a closed bottom such as thecan in FIG. 1.

The can material will depend in part of the active materials andelectrolyte used in the cell. A common material type is steel. Forexample, the can may be made of steel, plated with nickel on at leastthe outside to protect the outside of the can from corrosion. The typeof plating can be varied to provide varying degrees of corrosionresistance or to provide the desired appearance. The type of steel willdepend in part on the manner in which the container is formed. For drawncans the steel can be a diffusion annealed, low carbon, aluminum killed,SAE 1006 or equivalent steel, with a grain size of ASTM 9 to 11 andequiaxed to slightly elongated grain shape. Other steels, such asstainless steels, can be used to meet special needs. For example, whenthe can is in electrical contact with the cathode, a stainless steel maybe used for improved resistance to corrosion by the cathode andelectrolyte.

The cell cover can be metal. Nickel plated steel may be used, but astainless steel is often desirable, especially when the cover is inelectrical contact with the cathode. The complexity of the cover shapewill also be a factor in material selection. The cell cover may have asimple shape, such as a thick, flat disk, or it may have a more complexshape, such as the cover shown in FIG. 1. When the cover has a complexshape like that in FIG. 1, a type 304 soft annealed stainless steel withASTM 8-9 grain size may be used, to provide the desired corrosionresistance and ease of metal forming. Formed covers may also be plated,with nickel for example.

The terminal cover should have good resistance to corrosion by water inthe ambient environment, good electrical conductivity and, when visibleon consumer batteries, an attractive appearance. Terminal covers areoften made from nickel plated cold rolled steel or steel that is nickelplated after the covers are formed. Where terminals are located overpressure relief vents, the terminal covers generally have one or moreholes to facilitate cell venting.

The gasket is made from any suitable thermoplastic material thatprovides the desired sealing properties. Material selection is based inpart on the electrolyte composition. Examples of suitable materialsinclude polypropylene, polyphenylene sulfide,tetrafluoride-perfluoroalkyl vinylether copolymer, polybutyleneterephthalate and combinations thereof. Preferred gasket materialsinclude polypropylene (e.g., PRO-FAX® 6524 from Basell Polyolefins inWilmington, Del., USA) and polyphenylene sulfide (e.g., XTEL™ XE3035 orXE5030 from Chevron Phillips in The Woodlands, Tex., USA). Small amountsof other polymers, reinforcing inorganic fillers and/or organiccompounds may also be added to the base resin of the gasket.

The gasket may be coated with a sealant to provide the best seal.Ethylene propylene diene terpolymer (EPDM) is a suitable sealantmaterial, but other suitable materials can be used.

If a ball vent is used, the vent bushing is made from a thermoplasticmaterial that is resistant to cold flow at high temperatures (e.g., 75°C.). The thermoplastic material comprises a base resin such asethylene-tetrafluoroethylene, polybutylene terephthlate, polyphenylenesulfide, polyphthalamide, ethylene-chlorotrifluoroethylene,chlorotrifluoroethylene, perfluoro-alkoxyalkane, fluorinatedperfluoroethylene polypropylene and polyetherether ketone.Ethylene-tetrafluoroethylene copolymer (ETFE), polyphenylene sulfide(PPS), polybutylene terephthalate (PBT) and polyphthalamide arepreferred. The resin can be modified by adding a thermal-stabilizingfiller to provide a vent bushing with the desired sealing and ventingcharacteristics at high temperatures. The bushing can be injectionmolded from the thermoplastic material. TEFZEL® HT2004 (ETFE resin with25 weight percent chopped glass filler), polythlalamide (e.g., AMODEL®ET10011 NT, from Solvay Advanced Polymers, Houston, Tex.) andpolyphenylene sulfide (e.g., e.g., XTEL™ XE3035 or XE5030 from ChevronPhillips in The Woodlands, Tex., USA) are preferred thermoplasticbushing materials.

The vent ball itself can be made from any suitable material that isstable in contact with the cell contents and provides the desired cellsealing and venting characteristic. Glasses or metals, such as stainlesssteel, can be used. In the event a foil vent is utilized in place of thevent ball assembly described above (e.g., pursuant to U.S. PatentApplication Publication No. 2005/0244706), the above referencedmaterials may still be appropriately substituted.

Electrodes

The anode comprises a strip of lithium metal, sometimes referred to aslithium foil. The composition of the lithium can vary, though forbattery grade lithium, the purity is always high. The lithium can bealloyed with other metals, such as aluminum, to provide the desired cellelectrical performance or handling ease, although the amount of lithiumin any alloy should nevertheless be maximized and alloys designed forhigh temperature application (i.e., above the melting point of purelithium) are not contemplated. Appropriate battery gradelithium-aluminum foil, containing 0.5 weight percent aluminum, isavailable from Chemetall Foote Corp., Kings Mountain, N.C., USA.

Other anode materials may be possible, including sodium, potassium,zinc, magnesium and aluminum, either as co-anodes, alloying materials ordistinct, singular anodes. Ultimately, the selection of an appropriateanode material will be influenced by the compatibility of that anodewith LiI, the cathode and/or the ether(s) selected.

As in the cell in FIG. 1, a separate current collector (i.e., anelectrically conductive member, such as a metal foil, on which the anodeis welded or coated OR an electrically conductive strip running alongthe length of the anode) is not needed for the anode, since lithium hasa high electrical conductivity. By not utilizing such a currentcollector, more space is available within the container for othercomponents, such as active materials. Anode current collectors may bemade of copper and/or other appropriate high conductivity metals so aslong as they are stable when exposed to the other interior components ofthe cell (e.g., electrolyte), and therefore also add cost.

The electrical connection must be maintained between each of theelectrodes and the opposing terminals proximate to or integrated withthe housing. An electrical lead 36 can be made from a thin metal stripconnecting the anode or negative electrode to one of the cell terminals(the can in the case of the FR6 cell shown in FIG. 1). When the anodeincludes such a lead, it is oriented substantially along a longitudinalaxis of the jellyroll electrode assembly and extends partially along awidth of the anode. This may be accomplished embedding an end of thelead within a portion of the anode or by simply pressing a portion suchas an end of the lead onto the surface of the lithium foil. The lithiumor lithium alloy has adhesive properties and generally at least aslight, sufficient pressure or contact between the lead and electrodewill weld the components together. The negative electrode may beprovided with a lead prior to winding into a jellyroll configuration.The lead may also be connected via appropriate welds.

The metal strip comprising the lead 36 is often made from nickel ornickel plated steel with sufficiently low resistance (e.g., generallyless than 15 mΩ/cm and preferably less than 4.5 mΩ/cm) in order to allowsufficient transfer of electrical current through the lead and haveminimal or no impact on service life of the cell. A preferred materialis 304 stainless steel. Examples of other suitable negative electrodelead materials include, but are not limited to, copper, copper alloys,for example copper alloy 7025 (a copper, nickel alloy comprising about3% nickel, about 0.65% silicon, and about 0.15% magnesium, with thebalance being copper and minor impurities); and copper alloy 110; andstainless steel. Lead materials should be chosen so that the compositionis stable within the electrochemical cell including the nonaqueouselectrolyte. Examples of metals generally to be avoided but can bepresent as impurities in relatively minor amounts are aluminum, iron andzinc.

The cathode is in the form of a strip that comprises a current collectorand a mixture that includes one or more electrochemically activematerials, usually in particulate form. Iron disulfide (FeS₂) is apreferred active material although the invention is applicable to mostcathode materials that are stable with LiI and have a potential vs. Lithat is less than 2.8V, possibly including CuO, CuO₂ and oxides ofbismuth (e.g., Bi₂O₃, etc.). Notably, MnO₂ is not suitable because thesecathodes have a potential that is too high when compared to the I₂/I⁻redox couple.

In a LiFeS₂ cell, the active material comprises greater than 50 weightpercent FeS₂. The cathode can also contain one or more additional activematerials mentioned above, depending on the desired cell electrical anddischarge characteristics. More preferably the active material for aLiFeS₂ cell cathode comprises at least 95 weight percent FeS₂, yet morepreferably at least 99 weight percent FeS₂, and most preferably FeS₂ isthe sole active cathode material. FeS₂ having a purity level of at least95 weight percent is available from Washington Mills, North Grafton,Mass., USA; Chemetall GmbH, Vienna, Austria; and Kyanite Mining Corp.,Dillwyn, Va., USA. A more comprehensive description of the cathode, itsformulation and a manner of manufacturing the cathode is provided below.

The current collector may be disposed within or imbedded into thecathode surface, or the cathode mixture may be coated onto one or bothsides of a thin metal strip. Aluminum is a commonly used material. Thecurrent collector may extend beyond the portion of the cathodecontaining the cathode mixture. This extending portion of the currentcollector can provide a convenient area for making contact with theelectrical lead connected to the positive terminal. It is desirable tokeep the volume of the extending portion of the current collector to aminimum to make as much of the internal volume of the cell available foractive materials and electrolyte.

The cathode is electrically connected to the positive terminal of thecell. This may be accomplished with an electrical lead, often in theform of a thin metal strip or a spring, as shown in FIG. 1, althoughwelded connections are also possible. The lead is often made from nickelplated stainless steel. Still another embodiment may utilize aconnection similar to that disclosed in U.S. patent application Ser. No.11/439,835, which should publish on or after Nov. 29, 2007, and/or U.S.patent application Ser. No. 11/787,436, which should publish on or afterOct. 16, 2008, both of which are commonly assigned to the assignee ofthis application and incorporated by reference herein. Notably, to theextent a cell design may utilize one of these alternative electricalconnectors/current limiting devices, the use of a PTC may be avoided. Inthe event an optional current limiting device, such as a standard PTC,is utilized as a safety mechanism to prevent runaway discharge/heatingof the cell, a suitable PTC is sold by Tyco Electronics in Menlo Park,Calif., USA. Other alternatives are also available.

Separator

The separator is a thin microporous membrane that is ion-permeable andelectrically nonconductive. It is capable of holding at least someelectrolyte within the pores of the separator. The separator is disposedbetween adjacent surfaces of the anode and cathode to electricallyinsulate the electrodes from each other. Portions of the separator mayalso insulate other components in electrical contact with the cellterminals to prevent internal short circuits. Edges of the separatoroften extend beyond the edges of at least one electrode to insure thatthe anode and cathode do not make electrical contact even if they arenot perfectly aligned with each other. However, it is desirable tominimize the amount of separator extending beyond the electrodes.

To provide good high power discharge performance it is desirable thatthe separator have the characteristics (pores with a smallest dimensionof at least 0.005 μm and a largest dimension of no more than 5 μmacross, a porosity in the range of 30 to 70 percent, an area specificresistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5)disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and herebyincorporated by reference.

Suitable separator materials should also be strong enough to withstandcell manufacturing processes as well as pressure that may be exerted onthe separator during cell discharge without tears, splits, holes orother gaps developing that could result in an internal short circuit. Tominimize the total separator volume in the cell, the separator should beas thin as possible, preferably less than 25 μm thick, and morepreferably no more than 22 μm thick, such as 20 μm or 16 μm. A hightensile stress is desirable, preferably at least 800, more preferably atleast 1000 kilograms of force per square centimeter (kgf/cm²). For anFR6 type cell the preferred tensile stress is at least 1500 kgf/cm² inthe machine direction and at least 1200 kgf/cm² in the transversedirection, and for a FR03 type cell the preferred tensile strengths inthe machine and transverse directions are 1300 and 1000 kgf/cm²,respectively. Preferably the average dielectric breakdown voltage willbe at least 2000 volts, more preferably at least 2200 volts and mostpreferably at least 2400 volts. The preferred maximum effective poresize is from 0.08 μm to 0.40 μm, more preferably no greater than 0.20μm. Preferably the BET specific surface area will be no greater than 40m²/g, more preferably at least 15 m²/g and most preferably at least 25m²/g. Preferably the area specific resistance is no greater than 4.3ohm-cm², more preferably no greater than 4.0 ohm-cm², and mostpreferably no greater than 3.5 ohm-cm². These properties are describedin greater detail in U.S. Patent Publication No. 2005/0112462, which ishereby incorporated by reference.

Separator membranes for use in lithium batteries are often made ofpolypropylene, polyethylene or ultrahigh molecular weight polyethylene,with polyethylene being preferred. The separator can be a single layerof biaxially oriented microporous membrane, or two or more layers can belaminated together to provide the desired tensile strengths inorthogonal directions. A single layer is preferred to minimize the cost.Suitable single layer biaxially oriented polyethylene microporousseparator is available from Tonen Chemical Corp., available from EXXONMobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separatorhas a 20 μm nominal thickness, and Setela 16MMS grade has a 16 μmnominal thickness. Suitable separators with similar properties are alsoavailable from Entek Membranes in Lebanon, Oreg., USA.

Cell Construction and Manufacture

The anode, cathode and separator strips are combined together in anelectrode assembly. The electrode assembly may be a spirally wounddesign, such as that shown in FIG. 1, made by winding alternating stripsof cathode, separator, anode and separator around a mandrel, which isextracted from the electrode assembly when winding is complete. At leastone layer of separator and/or at least one layer of electricallyinsulating film (e.g., polypropylene) is generally wrapped around theoutside of the electrode assembly. This serves a number of purposes: ithelps hold the assembly together and may be used to adjust the width ordiameter of the assembly to the desired dimension. The outermost end ofthe separator or other outer film layer may be held down with a piece ofadhesive tape or by heat sealing. The anode can be the outermostelectrode, as shown in FIG. 1, or the cathode can be the outermostelectrode. Either electrode can be in electrical contact with the cellcontainer, but internal short circuits between the outmost electrode andthe side wall of the container can be avoided when the outermostelectrode is the same electrode that is intended to be in electricalcontact with the can.

The cell can be closed and sealed using any suitable process. Suchprocesses may include, but are not limited to, crimping, redrawing,colleting and combinations thereof. For example, for the cell in FIG. 1,a bead is formed in the can after the electrodes and insulator cone areinserted, and the gasket and cover assembly (including the cell cover,contact spring and vent bushing) are placed in the open end of the can.The cell is supported at the bead while the gasket and cover assemblyare pushed downward against the bead. The diameter of the top of the canabove the bead is reduced with a segmented collet to hold the gasket andcover assembly in place in the cell. After electrolyte is dispensed intothe cell through the apertures in the vent bushing and cover, a ventball is inserted into the bushing to seal the aperture in the cellcover. A PTC device and a terminal cover are placed onto the cell overthe cell cover, and the top edge of the can is bent inward with acrimping die to hold retain the gasket, cover assembly, PTC device andterminal cover and complete the sealing of the open end of the can bythe gasket.

With respect to the cathode, the cathode is coated onto a metallic foilcurrent collector, typically an aluminum foil with a thickness between18 and 20 μm, as a mixture which contains a number of materials thatmust be carefully selected to balance the processability, conductivityand overall efficiency of the coating. This coating consists primarilyof iron disulfide (and its impurities); a binder that is generally usedto hold the particulate materials together and adhere the mixture to thecurrent collector; one or more conductive materials such as metal,graphite and carbon black powders added to provide improved electricalconductivity to the mixture, although the amount of conductor dependsupon the electrical conductivity of the active material and binder, thethickness of the mixture on the current collector and the currentcollector design; and various processing or rheological aids that aredependent upon the coating method, the solvent used and/or the mixingmethod itself.

The following are representative materials that may be utilized in thecathode mix formulation: pyrite (at least 95% pure); conductor (PureBlack 205-110 from Superior Graphite Chicago, Ill., and/or MX15 fromTimcal Westlake, Ohio); and binder/processing aids(styrene-ethylene/butylenes-styrene (SEBS) block copolymer, such asg1651 from Kraton Polymers Houston, Tex., and Efka 6950 from Heerenveen,Netherlands). Small amounts of impurities may be naturally present inany of the aforementioned materials, although care should be taken toutilize the highest purity pyrite source available so as to maximize theamount of FeS₂ present within the cathode.

It is also desirable to use cathode materials with small particle sizesto minimize the risk of puncturing the separator. For example, FeS₂ ispreferably sieved through a 230 mesh (62 μm) screen before use or theFeS₂ may be milled or processed as described in U.S. Patent PublicationNo. 2005/0233214, which is incorporated by reference herein. Othercathode mix components should be carefully selected with eye towardchemical compatibility/reactivity and to avoid similarparticle-size-based mechanical failure issues.

The cathode mixture is applied to the foil collector using any number ofsuitable processes, such as three roll reverse, comma coating or slotdie coating. The methods of coating described in U.S. patent applicationSer. No. 11/493,314, which should publish on or after Jan. 31, 2008 andis incorporated by reference, could be used. One preferred method ofmaking FeS₂ cathodes is to roll coat a slurry of active material mixturematerials in a highly volatile organic solvent (e.g., trichloroethylene)onto both sides of a sheet of aluminum foil, dry the coating to removethe solvent, calendar the coated foil to compact the coating, slit thecoated foil to the desired width and cut strips of the slit cathodematerial to the desired length. The use of volatile solvents maximizethe efficiency of recovering such solvents, although it is possible toutilize other solvents, including aqueous-based compositions, in orderto roll coat the cathode mix described above.

After or concurrent with drying to remove any unwanted solvents, theresulting cathode strip is densified via calendering or the like tofurther compact the entire positive electrode. In light of the fact thatthis strip will then be spirally wound with separator and a similarly(but not necessarily identically) sized anode strip to form a jellyrollelectrode assembly, this densification maximizes loading ofelectrochemical material in the jellyroll electrode assembly. However,the cathode cannot be over-densified as some internal cathode voids areneed to allow for expansion of the iron disulfide during discharge andwetting of the iron disulfide by the organic electrolyte, as well as toavoid unwanted stretching and/or de-lamination of the coating.

Example 1

Six different solvent blends were prepared according to Table 1 below.Sample A is representative of the prior art, while Samples B and Ccontain insufficient amounts of DME and EME, respectively. All samplesutilized 0.75 molal lithium iodide as the solute.

TABLE 1 Solvent Blends Sample Vol. % DIOX Vol. % DME Vol. % EME A 65.035.0 0 B 75.0 8.3 16.7 C 75.0 16.7 8.3 D 55.0 30.0 15.0 E 65.0 17.5 17.5F 55.0 15.0 30.0

Example 2

Six separate lots of standard LiFeS₂ AA-sized batteries, as describedabove, were constructed using spirally wound electrodes oflithium-aluminum alloy and iron disulfide slurry coated onto an aluminumfoil collector separated by a polyethylene separator. The only variablebetween batteries was the choice of electrolyte, with each of the blendsfrom Example 1 being incorporated into a number of cells. Thesebatteries were then discharged under varying conditions as described inTables 2-4 below, with the lot designation corresponding to theelectrolyte samples in Example 1. Given statistical and other variables,any results within plus or minus approximately 5 to 10 percent of thebaseline is considered acceptable service performance.

The “signature test” is a sequential continuous drain test at the statedrate. When the battery reaches the set cut off point for that drain(typically between 0.9 and 1.1 V, so long as the same cutoff ismaintained throughout the entire test), it is allowed to rest for astandard period of time (typically, one hour) and then discharged at thenext lowest rate. All performance values are normalized to electrolytesample A, which represents the prior art.

TABLE 2 Relative performance at −40° C. on the signature test. Allvalues reported as percent service compared to Lot A Drain Rate (mA) LotA Lot B Lot C Lot D Lot E Lot F 1000 100 75 79 171 99 123 750 100 5 83517 89 4298 500 100 0 1 169 158 191 375 100 0 1 190 170 221 250 100 0 1181 199 219 200 100 0 2 177 203 216 150 100 0 72 170 204 211 100 100 0134 146 176 179

TABLE 3 Relative performance at 21° C. on the signature test. All valuesreported as percent service compared to Lot A Drain Rate (mA) Lot A LotB Lot C Lot D Lot E Lot F 1000 100 94 96 98 97 94  750 100 95 96 98 9794  500 100 96 96 99 98 95  375 100 96 96 99 98 95  250 100 97 96 99 9996  200 100 97 96 99 99 96  150 100 97 96 99 99 96  100 100 97 96 99 9996

TABLE 4 Relative performance on ANSI digital still camera test. Allvalues reported as percent service compared to Lot A Temperature Lot ALot B Lot C Lot D Lot E Lot F −20° C. 100 13 87 101 92 77  21° C. 100 8793  98 93 91

Features of the invention and its advantages will be further appreciatedby those practicing the invention, particularly with reference to theExamples, Figures, Tables and other information provided herein and anypatent references above necessary to better understand the invention areincorporated herein to that extent. In the same manner, it will beunderstood by those who practice the invention and those skilled in theart that various modifications and improvements may be made to theinvention without departing from the spirit of the disclosed concepts.The scope of protection afforded is to be determined by the claims andby the breadth of interpretation allowed by law.

1. An electrochemical cell comprising: an anode consisting essentiallyof lithium or a lithium alloy; a cathode comprising FeS₂; and anelectrolyte consisting essentially of at least one solute dissolved in asolvent blend of at least 40 vol. % DIOX, at least 10 vol. % of one ormore linear asymmetric ethers and an optional amount of DME; wherein thelinear asymmetric ethers have a structural formula selected from thegroup consisting of: R₁—O—CH₂—CH₂—O—R₂ and R₁—O—CH₂—CH(CH₃)—O—R₂ whereinR₁ and R₂ are alkyl, cyclic, aromatic or halogenated groups, the linearasymmetric ether has 7 or more carbon atoms and R₁≠R₂.
 2. Anelectrochemical cell according to claim 1, wherein the solute includesat least one selected from the group consisting of: lithium iodide,lithium trifluoromethanesulfonate, lithium bistrifluoromethylsulfonylimide, lithium perchlorate and lithium hexafluoroarsenate.
 3. Anelectrochemical cell comprising: an anode consisting essentially oflithium or a lithium alloy; a cathode comprising FeS₂; and anelectrolyte consisting essentially of at least one solute dissolved in asolvent blend of at least 40 vol. % DIOX, at least 10 vol. % of one ormore linear asymmetric ethers and an optional amount of DME; wherein thelinear asymmetric ethers have a structural formula selected from thegroup consisting of: R₁—O—CH₂—CH₂—O—R₂ and R₁—O—CH₂—CH(CH₃)—O—R₂ whereinat least 10 vol. % of DME is included in the solvent blend, R₁ is anethyl group and R₂ is a methyl group.
 4. An electrochemical cellaccording to claim 3, wherein the solute includes at least one selectedfrom the group consisting of: lithium iodide, lithiumtrifluoromethanesulfonate, lithium bistrifluoromethylsulfonyl imide,lithium perchlorate and lithium hexafluoroarsenate.